This invention relates to a combination reformer for natural gas feed fuel and fuel cell stack separator, in a solid oxide fuel cell configuration.
High temperature solid oxide electrolyte fuel cell generators are well known, and taught by U.S. Pat. Nos. 4,395,468 and 4,728,584 (Isenberg). There, tubular, solid oxide electrolyte fuel cells are placed in a generator chamber defined by an alumina board housing. In larger generators, insulating divider sheets are generally used between rows of cell bundles, which bundles usually contain from 12 to 36 or more fuel cells, and may also be used between individual rows of fuel cells, as taught in U.S. Pat. Nos. 4,876,163 and 4,808,491 (both Reichner).
In these high temperature fuel cells, air and fuel are processed to produce heat and electricity. Direct use of hydrocarbon fuels, such as methane, ethane, mixtures of hydrocarbons such as natural gas (CH.sub.3 +C.sub.2 H.sub.6), or alcohols such as ethyl alcohol can form carbon on the fuel cells and other components of the generator, and can reduce the efficiency of the fuel cells by blocking gas transport and by providing electrical short-circuit paths. These hydrocarbon fuel gases are generally mixed with water vapor and reformed as an initial step, that is, converted to CO and H.sub.2, through the use of a catalyst, usually platinum or nickel or compounds thereof. The reforming reaction is endothermic (requires a supply of heat) and is best performed at temperatures close to that of the solid oxide fuel cell operation (900.degree. C. to 1,000.degree. C.). Reforming fuel outside of the generator is undesirable, resulting in a loss of energy, as heat loss from the reformer and from connecting conduits, and making the system more expensive and complicated.
In U.S. Pat. No. 4,374,184 (Somers et al.), an attempt to solve this problem was made by in-situ reforming on a deliberately constructed inactive end of each tubular fuel cell. This however, cut down dramatically on active fuel cell area within the .cell stack. In U.S. Pat. No. 4,729,931 (Grimble), catalytic packing, for reformation of hydrocarbon fuel, is placed in a catalyst chamber on the outside of the fuel cell chamber and feeding into the side of a fuel inlet plenum. A reformable gaseous fuel is fed into the open top end of the catalyst chamber and mixed with a portion of spent fuel prior to reformation, and the reformed mixture is passed directly alongside the cells. In U.S. Pat. No. 4,808,491 (Reichner), hot exhaust gas used to heat the corners of the generator is passed through an optional reformer catalyst bed directly underneath the closed ends of the fuel cells.
With the methods described previously, in practical generators, it is difficult to transfer the heat necessary for the endothermic reforming reaction without creation of excessive temperature differences within the cell stack and in the reformer. Air flow to the cells must be increased beyond that required for reaction with the fuel, to prevent excessive temperature gradients. While these methods provide useful internal reforming, a better performing internal reformer is needed. It is a main object of this invention to provide such an internal reformer.